1. Field of the Invention:
The invention relates to a register-measuring system and, more particularly, to such a system which is of the type used in offset printing for checking the position of the printing plates.
In multi-color printing, several printing plates are required for the different color separations. Particular importance is attached in this regard to the position of the individual printing plates; in particular, the relative positions of the individual printing plates with regard to circumferential and side register, respectively, are of great importance for the quality of the printed product. Incorrect positioning of the printing plates leads to a reduction in quality. In general, therefore, efforts are in progress to create means by which it is possible to provide precise position-determination and, as a consequence thereof, precise positioning of the printing plates. In order to facilitate the positioning of the individual printing plates for the various color separations, use is made at present of, for example, cross-shaped register marks, so-called register crosses, on the individual printing plates. These register crosses are usually in the non-printed area of the printed sheet, for example, in the region of the edges or corners of the sheet. By means of these register crosses an adjustment of the position of the plates is possible, a relative superimposition of the individual register crosses being assessed or detected.
If the register crosses of the individual printing plates are not exactly superimposed relative to one another, the degree of deviation is usually established by an inspection on the part of the observer. Adjustment is then made to the printing plate in question and is repeated until the register crosses of all of the colors are exactly superimposed.
A disadvantage thereof is the time-consuming process of adjustment owing to the fact that the very thin lines (approx. 0.1 mm) are difficult to see, even when examined with a magnifying glass, and the estimation of the dimensional deviations in relation to one another (which is necessary within the range of 0.01 mm) is difficult and imprecise, so that the duration of the adjustment process depends upon the eyesight, skill and experience of the machine operator. Efforts are therefore also in progress to replace visual examination by an automatic reading device which determines the positions of register marks provided for this purpose.
Yet, even with an automatic reading device, great importance is attached to the formation of the register mark, because the reliability with which the reading device recognizes the register mark depends decisively on how the register mark is embodied or shaped.
2. Description of the Related Act
Automatic register-measuring systems operating on a photoelectric basis have become known heretofore from German Patent Nos. 32 48 795 and 32 26 078. These systems are on-line systems that perform measurements of the register marks during the production process.
The state of the art likewise includes television cameras that communicate with on-line and off-line systems and which scan and transmit register marks. This method, however, has the disadvantage that these systems are too complex and therefore too expensive. Moreover, the measuring systems belonging to the state of the art are predominantly of the stationary type and are difficult to install flexibly, respectively.